This invention relates to a carbon media for storage of hydrogen.
There is currently an intense interest in carbon materials due to their unique and novel properties. For instance, the carbon materials may be useful to achieve high hydrogen energy storage, for use in purification processes as well as for different applications within the electronical/pharmaceutical sector. The properties are sensitive to the micro-structure of the carbon material, which can be varied by the nanostructure ordering (graphitisation level). The nanostructure ordering spans from non-crystalline qualities such as conventional carbon black (furnace black, thermal black) to crystalline qualities such as graphite and novel carbon materials with graphitic structures. The nanostructure ordering can be described in terms of the distance between the graphite layers, which will vary from 3.40 xc3x85 for a ordered crystalline structure to 3.60 xc3x85 for non-crystalline materials.
The recent interest in carbon materials for use as a storage medium has mainly been focused on novel materials with graphitic structures where the degree of graphitisation and the introduction of rings other than hexagons in the network is of vital importance. Fullerenes are examples of novel graphitic structures where the introduction of 12 pentagons in the hexagonal network results in closed shells [1]. Carbon nanotubes is also an example of such graphitic structures, but only three of five possible kinds have ever been synthesised [3, 4, 5].
Recent interest in fullerenes and nanotubes is amongst other connected to their use in the field of hydrogen storage. Hence, for nanotubes a hydrogen storage of amazingly 75 wt % is reported [6]. If this is the case, it will probably represent the break-through concerning a practical hydrogen storage system for use in the transportation sector. It is indicated that future fuel cell cars using this storage technology may achieve a range of about 8000 km.
In the case of fullerenes, more than 7 wt % of reversibly added hydrogen is achieved [7, 8, 9]. Fullerenes has also been used in a solid phase mixture with inter-metallic compounds or metals to achieve high contents of hydrogen, i.e. 24-26 H atoms per fullerene molecule [10].
Flat graphitic material formed of stacks of two-dimensional sheets has high surface area for adsorption of guest elements and compounds. However, in such materials the adsorption process is probably limited by diffusion. The larger the graphitic domain, the slower the adsorption will be. Of potential interest would be highly graphitised materials where domains are small so is that the guest material would readily reach all the graphitic micro domains by percolation through the bulk carbon material. The accessibility to the micro-domains could be further enhanced if some or all the domains had topological disclination, preferably each domain having less or equal than 300xc2x0 disclination to provide cavities, or micro-pores, for the flow of guest material.
A common problem with the present methods for synthesizing these graphitic materials is the low production yield. The fullerenes are most often synthesized by [vaporising] vaporizing graphite electrodes via carbon-arc discharges in a reduced inert gas atmosphere. There has been reported a conversion rate into fullerenes of 10-15%, corresponding to a generation rate of nearly 10 grams per hour [11].
The carbon-arc method is also the most frequently used method for production of carbon nanotubes. Nanotubes yields of about 60% of the core material has been obtained at optimal conditions [2]. Still, the achieved production is in gram quantities.
Small unspecified amounts of open conical carbon structures are obtained by resistively heating a carbon foil and further condensing the carbon vapour on a highly-oriented pyrolytic graphite surface [3, 4]. The cone angles produced by this method was approximately 19xc2x0 [3], and 19xc2x0 as well as 60xc2x0 [4].
Resistive heating of a carbon rod, with further deposition on cooler surfaces was used to produce cones with apparent cone angles of approximately 39xc2x0 [5]. It can be shown from a continuous sheet of graphite that only five types of cones can be assembled, where each domain is uniquely defined by its topological disclination TD given by the general formula:
TD=Nxc3x9760 degrees, where N=0, 1, 2, 3, 4 or 5,
The structure of such graphitic domains can be grossly described as stacks of graphitic sheets with flat (N=0) or conical structures (N=1 to 5). Hence, two of these, holding cone angles of 83.6xc2x0 and 112.9xc2x0, has not been reported so far.
An object of this invention is to provide a carbon media for storage of hydrogen. This object is achieved by a media characterised in that it comprises known and novel crystalline or non-crystalline materials and that it is produced by a two-step plasma process. By changing the process parameters of the plasma process, the nanostructure ordering of the carbon material can be varied in such a way that the desired microstructure for optimum hydrogen storage is achieved. These microstructures may either be conventional carbon black graphitic carbon black and/or novel carbon materials such as cones, fullerenes or nanotubes.
In the one-step plasma process conventional carbon black or graphitic carbon black can be formed. A such process is described in for instance EP 0 636 162. The resulting carbon material may have a surface area (BET) of 5-250m2/g and a dibutyl phtalate absorption (DBP) of 40-175 ml/ 100 g.
In the two-step plasma process, a hydrocarbon feed material is sent through a plasma zone and becomes partly dehydrogenated in the first step to form polycyclic aromatic hydrocarbons (PAHs), and is then sent through second plasma zone to become completely dehydrogenated to form micro-domain graphitic materials in the second step. By micro-domain graphitic materials we mean fullerenes, carbon nanotubes, open conical carbon structures (also named micro-cones), Pat graphitic sheets, or a mixture of two or all of these. The novel part of the carbon material is open carbon micro-cones with total disclination degrees 60xc2x0 and/or 120xc2x0, corresponding to cone angles of respectively 112.9xc2x0 and/or 83.6xc2x0.
Another object of this invention is to provide a carbon media for storage of hydrogen comprising known and novel micro-domain materials, characterised in that the media is produced in industrial scale with large yield rates of up to above 90% by a one or two-step plasma process.
The invention also relates to use of the carbon media as a storage media for hydrogen.